New Vitrimer PCBs Can Be Recycled Many Times Over

A team of researchers at the University of Washington (UW) has developed a printed circuit board (PCB) based on a plastic material, called vitrimer, created less than two decades ago.

 

Vitrimer PCB being laminated in a prototype size heat press. 

 

The vitrimer epoxy creates a PCB with electrical and mechanical properties similar to standard FR-4 PCB material. This material enables an environmentally friendly recycling process that results in near-complete material recovery.

 

E-Waste: An Unwanted Byproduct of Modern Electronics

Electronic waste, or e-waste, is a good example of the law of unintended consequences in action. We design materials for low cost, durability, and reliability—attributes that make recycling very difficult. A PCB is a collection of toxic petroleum-based substances peppered with heavy metals with a semi-inert internal structure.

When an electronic device’s life is over, it may end up in a landfill where toxins leach off over time, poisoning the environment and wasting the valuable commodities contained therein. If it does end up being “recycled”, its fate is not what most of us would consider to be responsible recycling. It often lands in economically depressed areas, where subsistence laborers breathe in highly dangerous fumes as they burn the PCBs to recover small amounts of gold, silver, copper, and other metals.

 

Enter Vitrimer

PCBs are constructed with a structure consisting of a glass fiber mat impregnated with thermoset epoxy resin, called a prepreg layer. Prepreg sheets are laminated with a thin copper layer in which the electronic circuit is etched. Multiple layers are etched, drilled, and heat-laminated together to form a PCB stack. When the PCB layers are stacked, they are put into a high-pressure, high-temperature press, which finishes the curing process.

The UW team used a material called vitrimer, a plastic first announced in 2015. Vitrimer is a two-part thermoset epoxy that does not require dramatically new or exotic processing and promises a high degree of recyclability without harmful side effects. As a polymer, epoxy is constructed of long chains of hydrocarbons that intertwine during the curing process. For traditional polymers, curing is one way. Once the polymer chains are intertwined from full curing, they are stable and cannot be non-destructively separated.

This is where vitrimer materials differ. Like polymers, vitrimer chains wrap and intertwine when cured, but they can also unwrap when they make contact with heat or specific solvents. Vitrimer's ability to unwrap allows its chains to be cured, uncured, and recured repeatedly without degrading them, allowing for recovery during recycling.

 

Easier Storage, Familiar Process

Because traditional PCB prepreg is only partially cured, it requires careful storage and has a limited shelf life before it must be discarded. Vitrimer prepreg sheets can be fully cured, allowing easier and longer storage before being put into a finished vitrimer printed circuit board (vPCB).

Fully cured vitrimer prepreg can be bonded and recured with heat and pressure using the same press/ovens used in traditional PCB processing. As long as the board's copper layers are intact, cracked or warped vPCBs can be sent back into a thermal press and fully repaired because of their uncure/recure ability.

 

Untangling the Bonds for Better Recycling

 When recycling time comes around, the vitrimer epoxy base can be completely dissolved and reused.

 

Working vitrimer-based PCB (left) and recycled and recover components (right).

 

First, the components are removed through desoldering, and the copper is etched off. After that, the vPCB is treated with a solvent to separate the vitrimer into a jelly form, leaving the glass fiber mat and metal PCB traces to be easily recovered. The UW process has shown a 98% recovery and reuse of the vitrimer epoxy, a 100% recovery of the glass fiber and PCB metals, and a 91% recovery of the recycling solvent.

 

Will Vitrimer Work as a PCB?

In order to act as a viable replacement for standard PCB materials, vitrimer must have a similar cost and compatible electrical properties. It must also survive assembly and rework and meet mechanical and safety requirements.

In their extensive testing, the UW team built a 2.4-GHz transmitter using vPCB to simulate a typical IoT device. The vPCB featured electrical and mechanical characteristics very similar to FR-4, the most common PCB material in use. It also demonstrated durability under manufacturing and rework similar to FR-4. 

 

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All images used courtesy of the University of Washington.